U.S. patent number 4,326,919 [Application Number 06/143,240] was granted by the patent office on 1982-04-27 for nuclear core arrangement.
This patent grant is currently assigned to Westinghouse Electric Corp.. Invention is credited to Donald J. Hill.
United States Patent |
4,326,919 |
Hill |
April 27, 1982 |
Nuclear core arrangement
Abstract
A method and arrangement for utilizing mixed oxide fuel in the
core of a nuclear reactor controlled by rectilinearly movable
control rod elements which decreases the maximum single control rod
element worth in the core. Fuel elements containing the mixed oxide
fuel are positioned at discrete core locations, particularly at
those locations receiving high worth control rod elements, thereby
decreasing the worth of those control elements.
Inventors: |
Hill; Donald J. (O'Hara
Township, Allegheny County, PA) |
Assignee: |
Westinghouse Electric Corp.
(Pittsburgh, PA)
|
Family
ID: |
26840824 |
Appl.
No.: |
06/143,240 |
Filed: |
April 24, 1980 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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829703 |
Sep 1, 1977 |
|
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Current U.S.
Class: |
376/267; 376/236;
414/146; 976/DIG.5; 976/DIG.111 |
Current CPC
Class: |
G21C
1/00 (20130101); G21C 5/20 (20130101); Y02E
30/30 (20130101); Y02E 30/40 (20130101) |
Current International
Class: |
G21C
5/20 (20060101); G21C 5/00 (20060101); G21C
1/00 (20060101); G21C 019/20 (); G21G 001/02 () |
Field of
Search: |
;176/17,30,33,78
;414/146 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Walsh; Donald P.
Attorney, Agent or Firm: Levine; Edward L. Dermer; Z. L.
Parent Case Text
This is a continuation of application Ser. No. 829,703, filed Sept.
1, 1977, now abandon.
Claims
I claim:
1. A method of fueling a nuclear reactor having discrete core
positions, each position receiving a singular vertically disposed
bundled-rod fuel assembly, each said fuel assembly including a
plurality of fuel rods having nuclear fuel disposed within a
metallic cladding, at least some of said fuel assemblies bearing
fissionable plutonium nuclear fuel and at least some other of said
assemblies bearing fissionable nuclear fuel essentially free of
fissionable plutonium upon initial insertion into said core, said
reactor further having rectilinearly movable control elements, the
number of control elements being less than the number of fuel
assemblies, each said control element being insertable into a
selected one of said fuel assemblies in a corresponding one of said
core positions, each said control element, upon insertion, being
laterally bounded by said fuel assembly, said method
comprising;
(a) inserting a plutonium bearing fuel assembly into one of said
selected control element receiving core positions, said position
being the position of highest control element worth in the N-1
configuration based upon a core configuration having only fuel
assemblies essentially free of fissionable plutonium; and
(b) inserting said plutonium bearing fuel assemblies and said fuel
assemblies essentially free of fissionable plutonium into the
remaining core positions.
2. A core arrangement for a nuclear reactor utilizing fissionable
plutonium bearing fuel and having a plurality of discrete core
positions, each position receiving a singular vertically disposed
fuel assembly, each said fuel assembly including a plurality of
fuel rods having nuclear fuel disposed within a metallic cladding,
at least some of said fuel assemblies bearing fissionable plutonium
nuclear fuel and at least some other of said assemblies bearing
fissionable nuclear fuel essentially free of fissionable plutonium
upon initial insertion into said core, said reactor further having
rectilinearly movable control elements, each said control element
being insertable into one of said fuel assemblies in a
corresponding core position, said control element, upon insertion,
being laterally bounded by said fuel assembly, the number of
control elements being less than the number of fuel assemblies,
comprising:
(a) a plurality of said plutonium bearing fuel assemblies disposed
respectively in a plurality of said core positions;
(b) a plurality of said fuel assemblies essentially void of
fissionable plutonium fuel disposed respectively in another
plurality of said core positions; and
(c) at least one plutonium bearing fuel assembly disposed in the
discrete control element receiving core position having the highest
control element worth in the N-1 configuration based upon a core
configuration having only fuel assemblies essentially free of
fissionable plutonium.
3. A method of fueling a nuclear reactor core having discrete core
positions, each position receiving a singular vertically disposed
bundled-rod fuel assembly, each said fuel assembly including a
plurality of fuel rods having nuclear fuel disposed within a
metallic cladding, said core being controlled by rectilinearly
insertable control elements, the number of control elements being
less than the number of fuel assemblies, each said control element
being insertable into one of said fuel assemblies, each said
control element, upon insertion, being laterally bounded by said
fuel assembly, some of said fuel assemblies having fissionable
material consisting essentially of uranium and being essentially
free of plutonium, and some other of said fuel assemblies having
fissionable plutonium fuel, said method comprising:
(a) determining the control element receiving core position having
the highest reactivity in the N-1 configuration based upon a core
configuration having only uranium bearing fissionable fuel
assemblies essentially free of fissionable plutonium;
(b) inserting a plutonium bearing fuel assembly in said determined
position; and
(c) inserting said other plutonium bearing fuel assemblies and said
uranium bearing fuel assemblies initially essentially free of
plutonium into the balance of said core.
4. A nuclear reactor comprising:
(a) a reactor vessel having a head;
(b) a plurality of rectilinearly movable control elements
vertically insertable into said vessel from said head;
(c) a plurality of vertical coextending bundled-rod fuel assemblies
positioned within said vessel in discrete core positions, each said
position receiving a corresponding one of said fuel assemblies,
each said fuel assembly including a plurality of fuel rods having
nuclear fuel disposed within a metallic cladding, at least some of
said fuel assemblies bearing fissionable plutonium nuclear fuel and
at least some other of said assemblies bearing fissionable nuclear
fuel essentially free of fissionable plutonium upon initial
insertion into said core, the number of fuel assemblies being
greater than the number of control elements, some of said fuel
assemblies being sized and positioned to each removably receive and
laterally bound one of said control elements, at least one of said
control element receiving fuel assemblies being a plutonium bearing
assembly and being positioned at the position of the highest N-1
control element worth based upon a core configuration having only
fuel assemblies essentially free of fissionable plutonium upon
initial insertion into said core.
5. A core for a nuclear reactor controlled by selectively
positionable vertically disposed control elements, said core
comprising a plurality of discrete core positions, each position
receiving a singular vertical bundled-rod fuel assembly, each said
fuel assembly including a plurality of fuel rods having nuclear
fuel disposed within a metallic cladding, at least some of said
fuel assemblies bearing fissionable plutonium nuclear fuel and at
least some other of said assemblies bearing fissionable nuclear
fuel essentially free of fissionable plutonium upon initial
insertion into said core, some of said fuel assemblies sized and
positioned to removably receive and laterally bound a corresponding
one of said control elements, one of said control element receiving
fuel assemblies including fissionable plutonium fuel being disposed
at a preselected one of said discrete core positions so as to
decrease the worth of the control element otherwise having the
highest control element worth in the N-1 configuration based upon a
core configuration having only fuel assemblies essentially free of
fissionable plutonium upon initial insertion into said core.
6. A method of fueling a nuclear reactor having discrete core
positions, each position receiving a singular vertically disposed
fuel assembly, each said fuel assembly including a plurality of
fuel rods having nuclear fuel disposed within a metallic cladding,
said reactor further having rectilinearly movable control elements,
the number of control elements being less than the number fuel
assemblies, each said control element being insertable into a
selected one of said fuel assemblies in a corresponding one of said
core positions, each said control element, upon insertion, being
laterally bounded by said fuel assembly, said method
comprising:
(a) inserting an early cycle fuel assembly into each said core
position, each said early cycle fuel assembly bearing fissionable
nuclear fuel essentially free of plutonium;
(b) determining a fuel assembly loading pattern whereby a
fractional part of said early cycle assemblies are to be removed
from said core, at least some of the remaining assemblies are to be
shuffled to different positions within said core and new fuel
assemblies are to be inserted into the remaining fractional part of
said core, some of said new fuel assemblies to bear fissionable
plutonium nuclear fuel and some other of said new fuel assemblies
to bear fissionable nuclear fuel essentially free of plutonium upon
initial insertion into said core;
(c) analyzing said loading pattern to decide the control rod
receiving core position of highest worth in the N-1
configuration;
(d) inserting one of said plutonium bearing fuel assemblies into
said decided position; and
(e) shuffling and inserting assemblies in accordance with said
loading pattern in all other positions.
7. A method of determining a fuel assembly loading pattern for a
nuclear reactor having discrete core positions, each position
receiving a singular vertically disposed fuel assembly, each said
fuel assembly including a plurality of fuel rods having nuclear
fuel disposed within a metallic cladding, said reactor further
having rectilinearly movable control elements, the number of
control elements being less than the number of fuel assemblies,
each said control element being insertable into a selected one of
said fuel assemblies in a corresponding one of said core positions,
each said control element, upon insertion, being laterally bounded
by said fuel assembly, said method comprising:
(a) determining an early cycle fuel loading pattern whereby an
early cycle fuel assembly is inserted into each said core position,
each said early cycle fuel assembly bearing fissionable nuclear
fuel initially essentially free of plutonium; then
(b) determining the control element receiving core position of said
early cycle fuel loading pattern which has the highest control
element worth in the N-1 configuration, and then
(c) determining a subsequent cycle fuel loading pattern whereby a
subsequent cycle fuel assembly is inserted into each said core
position, some of said subsequent cycle fuel assemblies to bear
fissionable plutonium nuclear fuel, some other of said subsequent
cycle fuel assemblies to bear fissionable nuclear fuel initially
essentially free of plutonium, said subsequent cycle determination
being based upon the criteria that a plutonium bearing fuel
assembly is located at said determined position of highest control
element worth in the N-1 configuration.
8. A method of fueling a nuclear reactor having discrete core
positions, each position receiving a singular vertically disposed
fuel assembly, each said fuel assembly including a plurality of
fuel rods having nuclear fuel disposed within a metallic cladding,
said reactor further having recilinearly movable control elements,
the number of control elements being less than the number of fuel
assemblies, each said control element being insertable into a
selected one of said fuel assemblies in a corresponding one of said
core positions, each said control element, upon insertion, being
laterally bounded by said fuel assembly, said method
comprising:
inserting fuel assemblies into said core in accordance with the
determined subsequent cycle fuel loading pattern of claim 7.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to nuclear reactors, and more particularly
to reactor cores controlled by movable control rod elements and
utilizing mixed oxide fuel.
2. Description of the Prior Art
The use of fissionable plutonium fuel in nuclear reactors, commonly
referred to as mixed oxide fuel, is highly advantageous. Recycle of
plutonium fuel produced during the fissioning of other nuclear
fuels provides a useful energy resource. In this context, plutonium
fuel or mixed oxide fuel refers to a combination of fissionable
plutonium with other fissionable elements, such as a combination of
UO.sub.2 and PuO.sub.2. For example, a typical combination includes
uranium fuel having a U-235 concentration of approximately 0.2 to
1.1 percent by weight, and plutonium obtained from reprocessing
burned uranium fuel (first recycle plutonium) having approximately
a 4.2 weight percent concentration of plutonium. This invention, it
will be seen, is applicable to nuclear cores whenever plutonium, of
any significant concentration or type, is used, including plutonium
from successive recycles.
The inherent characteristics of mixed oxide fuel, however, limit
the use of the fuel because it results in increased reactor control
requirements. Plutonium has a high absorption cross section,
resulting in plutonium competition with the reactor control means,
typically control rod elements, for absorption of neutrons. This
competition results in a decrease in the worth of the control rod
elements. Further, insertion of plutonium in a core produces an
increased Doppler and moderator defect, requiring increased control
requirements.
As a result of the increased control requirements and decrease in
control rod element ability to control reactivity, reactor core
designers have provided core arrangements based upon positioning
the mixed oxide fuel away from the control rod elements. For
example, reactors have been proposed including bundled rod fuel
elements which position plutonium bearing fuel rods in the central
regions of fuel elements surrounded on their periphery by control
rod elements or bars. The peripheral fuel rods bear a more common
nuclear fuel, such as uranium. Also proposed have been
"checkerboard" arrangements wherein mixed oxide bearing fuel
elements are placed adjacent uranium bearing elements. With such
arrangements, the control rod elements are inserted only in the
uranium bearing assemblies. More recent arrangements have been
proposed which orient mixed oxide fuel only at the lower portions
of a core having top mounted control rod elements. This results in
the control rod elements being in the vicinity of the mixed oxide
fuel a decreased amount of time.
These prior art designs, based upon separating the control rod
elements or bars from the mixed oxide fuel, thereby
disadvantageously limit the amount of plutonium that can
effectively be used in a given core. Some have also required
additional control rod elements, beyond those required for a
primarily uranium bearing core, which is extremely costly.
Further, reactor control is critical from a safety viewpoint.
Throughout the nuclear power generating industry extreme care has
always been exercised to ensure conservative, redundant, and safe
designs. A typical approach to reactor control design has been to
assume that under postulated accident conditions, when insertion or
scram of the control rod elements is necessary, the control rod
element having the highest control element worth in terms of
reactivity is stuck out of the core. The worth of the control rod
elements in this configuration has been referred to as the "N minus
one" (N-1) control element worth. This hypothetical condition,
which serves as one basis for nuclear plant design, is used to
minimize the possibility of an area of the core maintaining
undesirable criticality with a control element stuck out of
position.
It is therefore desirable to provide a core which alleviates these
limitations heretofore brought about by plutonium or mixed oxide
utilization. It is also desirable to increase the amount of
plutonium that can be utilized in a given core, while increasing
safety margins through decreasing the potential for detrimental
effects under an accident condition.
SUMMARY OF THE INVENTION
This invention provides a core arrangement and method of fueling a
nuclear reactor utilizing fissionable plutonium or mixed oxide
fuel, typically as a combination of uranium and plutonium oxides.
It particularly addresses those reactor types that use movable
selectively positioned control rod elements as one form of reactor
control.
The invention provides placement of bundled-rod fuel elements
containing mixed oxide fuel in discrete core locations so as to
minimize the reactivity worth of the highest worth control element
in the N-1 configuration. Such placement can decrease the worth of
the otherwise highest worth control element so that it still
represents the highest control element worth, but at a lesser
value, or so that another control element in a given core position
becomes the control element of highest control element worth. This
other element, however, would then have a lower worth than the
previously highest worth control element in the N-1
configuration.
In contradistinction to the prior art, the invention provides for
the intentional positioning of preselected mixed oxide bearing fuel
elements in core locations such that those elements will receive a
control element. The mixed oxide bearing elements furthermore are
positioned to receive control elements of the highest worth when
assumed stuck and unable to enter the core.
This radical departure from the prior art teachings results in
increased mixed oxide utilization without unduly increasing the
control apparatus and cost in a given reactor. It also increases
safety margins by reducing the worth of the most reactive control
element that can be stuck out of the core under accident
conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages, nature, and additional features of this invention
will become more apparent from the following description taken in
connection with the accompanying drawings, in which:
FIG. 1 is an elevation view, in section, of a typical nuclear
reactor and internal structures;
FIG. 2 is a perspective view of a typical bundled-rod nuclear fuel
element receiving a control element of the spider type;
FIG. 3 is a simplified plan view, in partial section, taken at
III--III of FIG. 1;
FIG. 4 is a schematic illustration of a core arrangement; and
FIG. 5 is another schematic of another core arrangement.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown a reactor vessel 10 housing
a nuclear reactor core 12. The core 12 includes a plurality of
parallel and coextending bundled-rod fuel elements 14, also known
as fuel assemblies, supported vertically by structure within the
vessel 10. The vessel 10 is sealed at the top by a head 16 from
which there is supported control element drive mechanisms 18 which
selectively position control elements 20 above and within some of
the fuel elements 14. During operation a reactor coolant fluid,
such as water, is typically pumped into the vessel through a
plurality of inlet nozzles 22, passes downward through an annular
region 24 between the vessel and a core barrel 23 and thermal
shield 25, turns in the vessel lower plenum 26, passes upwardly
through the core 12, and exits through a plurality of outlet
nozzles 28. The heat energy which the core imparts to the coolant
is transferred in heat transfer apparatus (not shown) typically for
the ultimate purpose of electrical power generation.
A typical fuel element 14 of the bundled-rod type is shown in
greater detail in FIG. 2. It includes a plurality of parallel and
coextending fuel rods 30, each of which includes nuclear fuel
pellets 32 stacked within a sealed metallic cladding 34. The fuel
rods 30 are primarily supported by upper 36 and lower 38 nozzles
and by grid structures 40 spaced along the element length. The
element is shown receiving a control element 20 of the "spider"
type, including a plurality of cylindrical control rods 21,
although plates, bars, singular rods, and so forth, can be used
with varying element configurations. The control element 20 is
comprised of a material having a high neutron absorption cross
section, such as boron carbide, tantalum, a combination of
silver-indium and cadmium, or many others well known in the art. It
is to be understood that while an open-lattice or grid-type fuel
element is shown, the teachings herein are applicable to other fuel
element structures, including those referred to as ducted elements
used in many reactor types, such as liquid metal cooled fast
breeder reactors.
FIG. 3 shows that the fuel elements 14 are disposed in core
locations in a regularly patterned array. The letters A through O
and numerals 1 through 15 are herein utilized to reference a given
core position (A-1, B-2, etc.). Typically cores 12 are
symmetrically arranged in quadrants or other geometric
configurations such that elements in locations, for example, J-6,
J-10, F-6, and F-10 experience similar operating characteristics
and compositions. A fuel element 14 is burned within a given core
position for a period of time, and is then removed from, or
reshuffled within, the core 12 during refueling operations. While a
given fuel element 14 can reside in several core positions
throughout its useful life, such as during three or more separate
core cycles, the control element 20 positions are typically fixed.
The control element positions in the exemplary core shown are
depicted by the solid circles in the Figure. Typically, as shown,
the control element positions are also symmetrically arranged.
The reactivity worth of the control elements 20 for a core 12 can
be determined by calculational methods and devices well known in
the art. The determinations can be performed by computer or other
calculational means. Knowledge of individual and total control
element reactivity worths is of vital importance in providing
necessary core control means, such as the elements 20, as well as
other neutron poisons and additionally in providing fluid
moderator, flow rate, density and composition requirements.
Although the basis for determination of, and the relative value of,
total and individual control element worths is herein presented in
relation to an all uranium core, it will be understood that such
determinations can be made on other bases, such as an assumed mixed
oxide and uranium core having uranium fuel elements at control rod
positions.
The reactivity worth of the control elements 20 in a core 12 is
typically discussed in terms of "total element worth" and "usable
element worth." Throughout this description and the appended
claims, "total control element worth" refers to the reactivity
worth of all the control elements of a given reactor in a given
core arrangement. "Useable control element worth" refers to the
reactivity worth in the same reactor with the most reactive control
element stuck out of the core. This is also referred to as the "N-1
control element worth," or the core in an "N-1 configuration." The
N-1 configuration, with the most reactive control element assumed
to be stuck out of the core, is the basis upon which core shutdown
margin and reactor safety requirements are typically based. As core
arrangements are typically symmetrical in two, four, eight, or
other numbers of segments, the control element of highest worth can
actually represent a plurality of control element positions, for
example, two control elements in each of the four quadrants of the
exemplary core 12, combining so as to provide eight symmetrical and
substantially similar positions. For purposes of design and
analysis, however, only one control element is assumed to be
stuck.
The disadvantageous effect on control requirements when plutonium
or mixed oxide fuel is introduced into a uranium or other type core
12 has in the past dictated that the mixed oxide fuel is in some
manner separated or spaced from the control elements 20. This
invention, however, provides for placement of mixed oxide bearing
fuel elements 14 specifically in those positions which otherwise
would have had the highest rod worths in an N-1 configuration.
Preferably fuel elements bearing the highest mixed oxide or
plutonium concentrations within a given core are so positioned. In
the exemplary core having eight symmetrical locations, eight fuel
elements are so positioned. This orientation decreases the stuck
control element worth of the otherwise highest worth control
element to a value which makes it no longer the highest worth
control element, or which reduces its incremental worth to a value
above the next highest worth control element, but less than its
previous worth.
The following example will better provide a description of the
advantages of the invention. It is to be understood, however, that
the example is merely illustrative, and that the invention can
effectively be practiced in many differing reactors and core
arrangements, utilizing various quantities and combinations of
plutonium fuel.
FIG. 4 shows the loading pattern for the exemplary all uranium
core. The numerals in the box at the lower right of selected
element positions identify the core regions. The locations
identified as "5" identify those uranium bearing fuel elements 14
initially placed in the core in the fifth operating cycle;
similarly those positions identified as "4" represent uranium fuel
elements initially placed in the core in the fourth operating
cycle. The letter "F" denotes fresh uranium bearing fuel elements.
These fresh elements are initially inserted into the core in the
sixth operating cycle. At this point in the core life, only fuel
elements initially bearing uranium, as opposed to plutonium, have
been utilized. Each fuel element ultimately resides in the core in
three different positions, and for three operating cycles, with
minor exceptions. It will be understood, however, that the
invention can beneficially be applied to core arrangements having
any number of fuel regions with fuel elements inserted for any
number of operating cycles.
Table I lists the usable control element worth and the stuck
control element worth for each control element receiving core
position for the all-uranium core of FIG. 4. The table is based
upon hot full power conditions at the end of life (EOL).
TABLE I ______________________________________ Usable Individual
Control Element Stuck Control Worth Element Worth Core
Configuration %.DELTA..rho. %.DELTA..rho.
______________________________________ N-1 Control Elements In
Element E-11 Stuck Out 6.41 0.90 Element B-10 Stuck Out 5.96 1.35
Element H-08 Stuck Out 7.20 0.11 Element F-08 Stuck Out 7.28 0.03
Element D-08 Stuck Out 7.25 0.06 Element E-09 Stuck Out 7.27 0.04
Element C-09 Stuck Out 6.63 0.68 Element D-10 Stuck Out 6.77 0.54
______________________________________
From Table I it can be seen that the control element located in
position B-10 is the most reactive control element stuck out of the
core. This conclusion also applies to the seven other control rods
at locations symmetric with B-10 in the exemplary core (F-14, J-14,
N-10, N-6, J-2, F-2, and B-6).
FIG. 5 shows the loading pattern of the core of FIG. 4 after a
number of operating cycles. Specifically, the all-uranium core of
FIG. 4 had been burned, and in the next operating cycle, some
all-uranium and some mixed oxide plutonium bearing elements had
been loaded into the core. The core was operated, the fuel burned,
and then a new region of some plutonium and some all-uranium
elements were inserted. This was continued to the point shown in
FIG. 5 and identified by the identification blocks noted thereon.
Accordingly, the core locations without hatching represent fuel
elements bearing uranium fuel as opposed to mixed oxide fuel, when
initially inserted into the core. Those identified as "8" have been
burned twice; those identified as "9" have been burned once, and
those identified as "10" and "F" are fresh at the beginning of the
cycle. The nomenclature similarly identifies those fuel elements
comprising mixed oxide fuel when initially placed in the core. The
mixed oxide elements have also been identified by cross
hatching.
As shown in FIG. 5, mixed oxide plutonium bearing fuel elements
have been deliberately positioned at core location B-10 and the 7
other symmetric locations, including location F-14. In particular,
fresh plutonium bearing elements have been so positioned.
Table II lists the usable control element worth and the stuck
control element worth for each control rod-receiving-core-position
for the core of FIG. 5. Table II is also based upon hot full power
conditions at end of life, i.e., after operating the FIG. 5
core.
TABLE II ______________________________________ Usable Individual
Control Element Stuck Control Worth Element Worth Core
Configuration %.DELTA..rho. %.DELTA..rho.
______________________________________ N-1 Control Elements In
Element E-11 Stuck Out 6.06 0.96 Element B-10 Stuck Out 6.09 0.93
Element H-08 Stuck Out 6.94 0.06 Element F-08 Stuck Out 6.97 0.05
Element D-08 Stuck Out 6.91 0.11 Element E-09 Stuck Out 6.95 0.07
Element C-09 Stuck Out 6.36 0.66 Element D-10 Stuck Out 6.47 0.55
______________________________________
From Table II it can be seen that the worth of the previously most
reactive stuck control element position, B-10, has been decreased
from 1.35% .DELTA.P for the all-uranium core to 0.93% .DELTA.P for
the mixed oxide core. It can also be noted that the worth of the
control element at core locations E-11, and for any symmetric
location, has increased from 0.90% .DELTA.P to 0.96% .DELTA.P.
Consequently, this element (E-11) is now the most reactive rod
potentially stuck out of the core at hot full power conditions. Its
N-1 worth, however, is substantially less than that of the rod at
position B-10 in the all-uranium core. It should be noted that
beneficial application of the invention need not be limited to only
the most reactive stuck control element position. For example,
placement of mixed oxide fuel in position E-11, in addition to
position B-10, will further decrease the highest individual stuck
control element worth in the core. Although the incremental benefit
to be obtained from such successive plutonium placement is, in this
example, rather small, in other core configurations the benefits
can be substantial. It will further be seen that there is, at hot
full power conditions, a relatively slight difference between the
worth of the control element located at position E-11 and the worth
of the control element located at position B-10. However, at hot
zero power conditions, the point at which shutdown margin is
typically determined, this condition changes.
Table III present a summary of control element worths at hot zero
power conditions for the exemplary all-uranium core arrangement and
the mixed core arrangement.
TABLE III ______________________________________ Control Element
Stuck Control Worth in Element Worth Core Configuration Core
%.DELTA..rho. %.DELTA..rho. ______________________________________
All Uranium Core EOL Cycle, HZP All Control Elements In Total -
6.95 -- N-1 Control Elements Element E-11 Stuck Out N-1 - 6.26 0.69
Element B-10 Stuck Out N-1 - 5.59 1.36 Core with 48 Pu Fueled
Assem- blies at EOL Cycle, HZP All Control Elements In Total - 6.79
-- N-1 Control Elements Element E-11 Stuck Out N-1 - 5.86 0.93
Element B-10 Stuck Out N-1 - 5.86 0.93
______________________________________
From Table III it can be seen that the N-1 control element worth of
the operative control elements for the mixed oxide arrangement
increases relative to the all-uranium core where a mixed oxide fuel
element is used at the N-1 core location, (It should be noted that
it is merely coincidental in this example that the N-1 control
element worths for E-11 both element and element B-10 stuck out of
the core are identical.) Thus, by deliberately placing plutonium
fuel at the most reactive stuck control element location as
determined for the all-uranium core, the usable control element
worth is considerably higher in the plutonium core. It can also be
seen that while the usable control element worth has increased, the
total control element worth has decreased.
Table IV presents a shutdown margin summary for the all uranium and
the mixed oxide core arrangements. It includes parameters typically
included in a shutdown margin analysis.
TABLE IV
__________________________________________________________________________
EOL Cycle 10, All U Core EOL Cycle 10, Mixed Oxide Item Shutdown
Margin, %.DELTA..rho. Core Shutdown Margin %.DELTA..rho.
__________________________________________________________________________
Control Requirements Power Defect 1.78 1.86 Redistribution 0.85
0.85 Void 0.05 0.05 Maneuvering Band & Bite 0.30 0.30 Total
Requirements 2.98 3.06 Control Element Worths N-1 Element Worth
5.59 5.86 Less Uncertainty 5.03 5.27 10% uncertainty for U core 13%
uncertainty for core with Pu fuel Steambreak Shutdown Requirement
1.72 1.69 Excess Shutdown Margin 0.33 0.52
__________________________________________________________________________
From Table IV it can be seen that the power defect, which is the
sum of the Doppler and moderator defect, did increase in the mixed
oxide core. However, the worth of all conrol elements in the core
with the most reactive element stuck out of the core with or
without conservative uncertainties, is still higher than the all
uranium core. In addition, by utilizing mixed oxide fuel at the
most reactive control element position, the reactivity required to
remain subcritical, for example, in an assumed steambreak accident,
is actually reduced. It can also be seen from Table IV that by
taking the N-1 control element worth, less conservative
uncertainties, less control requirements, less the steam-break
shutdown requirement, that the all uranium core has an excess
shutdown margin of 0.33% in reactivity while the plutonium core has
an excess shutdown margin of 0.52% in reactivity. Thus, the
plutonium fuel core has approximately 0.2% .DELTA.P more shutdown
margin than the all uranium core.
It should also be noted that in certain core configurations a fuel
element having the highest plutonium concentration among the core
elements would not be placed at a control element position. Other
factors must be considered including, for example, the effect upon
core power distribution. In the exemplary core arrangement the
control element at position B-10, having the highest stuck element
worth, is located in the peripheral core region which typically
receives fresh, or unburned, fuel elements. Had the highest stuck
control element worth control element been in, for example, the
intermediate core region (region 9 of FIG. 5), a once-burned
plutonium-bearing fuel element would be positioned to receive the
control element. If a fresh mixed oxide fuel element were placed in
the intermediate region it could result in an excess of reactivity
in that position causing local power peaking.
While the benefits to be achieved from utilization of this
invention have been illustrated through use of a singular example,
it will be apparent that the teaching can be equally applied to
many reactor and core types and arrangements. It therefore is to be
understood that within the scope of the appended claims, the
invention may be practiced other than as specifically
described.
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